Magnetic Compass (TMC)
medium
6 models total
A transmitting magnetic compass reads heading from a card aligned by the earth's own magnetic field, with no electrical power needed to keep pointing north; that independence is exactly why SOLAS still requires one even on ships fitted with a gyrocompass.
Read more — Magnetic Compass (TMC) explained ▾
What sets this instrument apart
A gyrocompass finds true north electrically, by sensing the earth's rotation, and needs power and settling time after a blackout before it can be trusted again. A magnetic compass needs neither: its card, floating in a liquid-filled bowl, aligns itself with the local magnetic field the moment it is free to move. That reliability under total power loss is why the magnetic compass remains a mandatory, physically separate piece of equipment rather than a redundant gyro repeater. Its reading is magnetic heading, not true heading, and it must be corrected for the ship's own magnetism before it means anything to the navigator.
Main components
Compass bowl and card
The card, carrying the compass rose and pivoted on a jewelled bearing, floats in a mixture of distilled water and alcohol chosen to stay fluid across the ship's operating temperature range and to damp out the card's swing in a seaway.
Binnacle and correctors
The bowl sits gimballed inside a binnacle that also carries the compensating magnets, the Flinders bar for correcting heeling error, and quadrantal spheres that counter the ship's induced magnetism. These are set during a compass swing and left undisturbed until the next one.
Transmitting unit
A separate sensor, usually a flux-gate or an optical pickup reading the card's position, converts the magnetic heading into an electrical signal for repeaters, the autopilot and the integrated bridge system — this is the "transmitting" part of a TMC, distinct from the analogue standard compass that is read by eye alone.
Regulations and class
SOLAS Chapter V regulation 19 requires a magnetic compass, or a means of determining heading by magnetic compass, on ships regardless of the navigation equipment otherwise fitted, along with a means of correcting heading for error. The compass must be adjusted by a qualified compass adjuster after major structural or magnetic changes near its position, and the resulting deviation card is kept on the bridge and checked against the log at survey. Class and flag state both expect the deviation card to be current, not a document that has quietly outlived a refit.
Typical faults
| Fault | Consequence |
|---|---|
| Deviation table left unrenewed after steelwork or new equipment near the compass | Bridge steers to an error the card no longer describes correctly |
| Bubble forming in the compass bowl from a slow liquid leak or temperature cycling | Card sticks or swings erratically, especially in a seaway |
| Gimbal wear or corrosion | Card no longer stays level as the ship rolls, degrading reading accuracy |
| Transmitting unit failure or misalignment with the card | Autopilot and repeaters receive a heading that disagrees with the compass actually read on the binnacle |
What to look for in a supplier
- Compensation range of the binnacle's correctors relative to the ship's expected magnetic disturbance
- Transmitting unit output format matched to the bridge system already installed (synchro versus digital interfaces)
- Ease of access for a compass adjuster to swing and correct the installation without dismantling the binnacle
- Liquid and bearing serviceability without full compass replacement
Practice
Officers should compare the standard compass reading to the gyro heading each watch as a matter of routine, not only when the gyro alarms, since a slow transmitting-unit drift produces no alarm at all.
Anything ferrous placed near the binnacle — a tool box, a phone, a replacement fire extinguisher — can shift the deviation enough to matter; the compass adjuster's card assumes the binnacle's surroundings stay as they were on swing day.
3 manufacturers · 6 models
Cassens & Plath
3
Reflecta II TMC
unverified
Card diameter (mm)
165
IMO Compliant
ja
Illuminated
ja
Common Failures & Inspection Points
- Sensor drift, damage or loss of reference caused by ageing, magnetic influence, antenna issues or internal faults can produce unstable or incorrect heading or position information
- Control or interface failure caused by electronics, cabling or network faults can create steering alarms, rejected inputs or loss of automatic control
- Power-supply interruption caused by loose connections, protection trips or internal failure can result in resets or total loss of indication
- Incorrect calibration or configuration after maintenance can show as persistent offset, poor course keeping or disagreement with independent references
- Mechanical wear in connected steering components where applicable can cause slow response, hunting, leakage or increased steering error
Service: Compare indications with independent references, inspect sensors and cable routes, verify power supplies and alarms, and test changeover or backup arrangements where fitted. For heading-control equipment, examine the connected steering feedback and response as well as the bridge controller. Refer to manufacturer documentation for exact calibration and acceptance values.
Spare Parts: Useful spares include maker-approved fuses, interface modules, power-supply parts, display or control modules, connectors and sensor accessories appropriate to the installed system.
Use Cases: Merchant, passenger, offshore and specialist vessels requiring heading, position, speed or automatic steering information at the bridge.
Neptune TMC
unverified
Card diameter (mm)
130
IMO Compliant
ja
Illuminated
ja
Common Failures & Inspection Points
- Loss of electrical, hydraulic, pneumatic, or other service supply caused by a tripped protection device, leakage, blockage, or upstream fault results in failed starting or loss of function
- Mechanical wear, corrosion, fouling, or contamination of moving and wetted parts causes increased noise, leakage, sticking, vibration, or reduced performance
- Seal, gasket, hose, cable, or connection deterioration from age, heat, vibration, or environmental exposure produces visible leakage, intermittent signals, or loss of pressure
- Sensor, switch, actuator, relay, or control-system faults can cause incorrect indication, nuisance alarms, failed automatic sequences, or loss of remote control
- Incorrect adjustment, assembly, or maintenance can lead to unstable operation, recurrent alarms, or performance that differs from the vessel's normal baseline
Service: Inspect the unit for leakage, corrosion, contamination, loose connections, abnormal noise or vibration, condition of guards and mountings, service supplies, controls, alarms, and functional performance. Use the vessel maintenance history to identify deterioration trends and refer to the manufacturer documentation for exact settings, tolerances, test values, and overhaul criteria.
Spare Parts: Keep the manufacturer-recommended seals and gaskets, filters where fitted, fuses or protection items, common sensors and switches, and critical actuator, control, or wear components appropriate to the installed configuration. Exact part numbers and quantities must come from the manufacturer spare-parts documentation.
Use Cases: Used on vessel types fitted with position & heading equipment, including merchant, offshore, passenger, fishing, or workboat installations where this specific magnetic compass (tmc) duty is required.
Standard 135 TMC
unverified
Card diameter (mm)
135
IMO Compliant
ja
Common Failures & Inspection Points
- Sensor, antenna or transducer fouling, damage or misalignment causes unstable, missing or inaccurate data
- Power-supply or processor faults cause resets, blank displays or complete loss of indication
- Cable, connector or network faults cause intermittent data or loss of outputs to dependent bridge systems
- Incorrect configuration or calibration causes persistent disagreement with independent references
- Display, storage or software faults cause frozen values, alarms or unavailable navigation functions
Service: Inspect sensors, antennas, transducers, cabling, power supplies, displays and data interfaces. Compare information with independent references and review alarm history. Verify distribution to dependent bridge systems and confirm approved backup arrangements. Refer to the manufacturer documentation for the exact figure for calibration, alignment and performance criteria.
Spare Parts: Carry fuses, interface cables, approved power supplies, sensor or antenna parts and supported display or processor modules, together with backed-up configuration data where applicable.
Use Cases: Navigation, pilotage, collision avoidance and bridge-system integration.
Observator
2
Royal 135 TMC
unverified
Card diameter (mm)
135
IMO Compliant
ja
Illuminated
ja
Common Failures & Inspection Points
- Sensor, antenna or transducer fouling, damage or misalignment causes unstable, missing or inaccurate data
- Power-supply or processor faults cause resets, blank displays or complete loss of indication
- Cable, connector or network faults cause intermittent data or loss of outputs to other bridge systems
- Incorrect configuration or calibration causes persistent disagreement with independent references
- Display or software faults cause frozen values, alarms or unavailable functions
Service: Inspect sensors, antennas, transducers, cabling, power supplies, displays and data interfaces. Compare information with independent references and review alarm history. Verify distribution to dependent bridge systems. Refer to the manufacturer documentation for the exact figure for calibration, alignment and performance criteria.
Spare Parts: Carry fuses, interface cables, approved power supplies, sensor or antenna parts and supported display or processor modules.
Use Cases: Navigation, pilotage, collision avoidance and bridge-system integration.
Super Yacht 100 TMC
unverified
Card diameter (mm)
100
IMO Compliant
ja
Common Failures & Inspection Points
- Sensor drift, damage or loss of reference caused by ageing, magnetic influence, antenna issues or internal faults can produce unstable or incorrect heading or position information
- Control or interface failure caused by electronics, cabling or network faults can create steering alarms, rejected inputs or loss of automatic control
- Power-supply interruption caused by loose connections, protection trips or internal failure can result in resets or total loss of indication
- Incorrect calibration or configuration after maintenance can show as persistent offset, poor course keeping or disagreement with independent references
- Mechanical wear in connected steering components where applicable can cause slow response, hunting, leakage or increased steering error
Service: Compare indications with independent references, inspect sensors and cable routes, verify power supplies and alarms, and test changeover or backup arrangements where fitted. For heading-control equipment, examine the connected steering feedback and response as well as the bridge controller. Refer to manufacturer documentation for exact calibration and acceptance values.
Spare Parts: Useful spares include maker-approved fuses, interface modules, power-supply parts, display or control modules, connectors and sensor accessories appropriate to the installed system.
Use Cases: Merchant, passenger, offshore and specialist vessels requiring heading, position, speed or automatic steering information at the bridge.
Lilley & Gillie
1
B-200 TMC
unverified
Card diameter (mm)
165
IMO Compliant
ja
Illuminated
ja
Common Failures & Inspection Points
- Loss of electrical, hydraulic, pneumatic, or other service supply caused by a tripped protection device, leakage, blockage, or upstream fault results in failed starting or loss of function
- Mechanical wear, corrosion, fouling, or contamination of moving and wetted parts causes increased noise, leakage, sticking, vibration, or reduced performance
- Seal, gasket, hose, cable, or connection deterioration from age, heat, vibration, or environmental exposure produces visible leakage, intermittent signals, or loss of pressure
- Sensor, switch, actuator, relay, or control-system faults can cause incorrect indication, nuisance alarms, failed automatic sequences, or loss of remote control
- Incorrect adjustment, assembly, or maintenance can lead to unstable operation, recurrent alarms, or performance that differs from the vessel's normal baseline
Service: Inspect the unit for leakage, corrosion, contamination, loose connections, abnormal noise or vibration, condition of guards and mountings, service supplies, controls, alarms, and functional performance. Use the vessel maintenance history to identify deterioration trends and refer to the manufacturer documentation for exact settings, tolerances, test values, and overhaul criteria.
Spare Parts: Keep the manufacturer-recommended seals and gaskets, filters where fitted, fuses or protection items, common sensors and switches, and critical actuator, control, or wear components appropriate to the installed configuration. Exact part numbers and quantities must come from the manufacturer spare-parts documentation.
Use Cases: Used on vessel types fitted with position & heading equipment, including merchant, offshore, passenger, fishing, or workboat installations where this specific magnetic compass (tmc) duty is required.